Understanding Klein's attack on RC4

Code:

Suppose w key streams were generated by RC4 using
packet keys with a fixed root key and different initialization
vectors. Denote by K_u = (K_u[0], ..., K_u[m]) = (IV_u||Rk) the
u-th packet key and by X_u = (X_u[0],...,X_u[m-1]) the first m
bytes of the uth key stream, where 1 <= u <= w. Assume that an
attacker knows the pairs (IV_u, X_u) - we shall refer to them as
samples - and tries to find Rk.

I've been reading this paragraph over and over, and I just don't get it. I think w is the amount of key streams, K is the key, K_u specifies which key stream it's a part of... but what is X?